1. Field of the Invention
The present invention relates to a manufacturing method of a silicon film having crystallinity or a silicon compound film (for instance, a SiGe semiconductor film) having crystallinity. For example, the invention can be applied to manufacture of a thin-film transistor.
2. Description of the Related Art
The thin-film transistor (hereinafter abbreviated as “TFT”) is known which uses a thin-film semiconductor. This is formed by using a thin-film semiconductor, particularly a silicon semiconductor film, that is formed on a substrate. While the TFT is used in various kinds of integrated circuits, it now attracts much attention particularly as a switching element provided for each pixel of an active matrix liquid crystal display device and a driver element formed in peripheral circuit sections of the same. The TFT now attracts much attention also as a device indispensable for multilayered integrated circuits (three-dimensional ICs).
As for a silicon film used in the TFT, it is simple and convenient to use an amorphous silicon film formed. However, the electrical characteristics of a resulting TFT are far lower than those of a TFT using a single crystal semiconductor for semiconductor integrated circuits. Therefore, at present, this type of TFT can be used for only limited purposes such as a switching element of an active matrix circuit. A silicon thin film having crystallinity may be used to improve the characteristics of the TFT.
Silicon films having crystallinity other than a single crystal silicon film are called a polysilicon film, a microcrystal silicon film, etc. Such a silicon film having crystallinity can be obtained by forming an amorphous silicon film and crystallizing it by heating (thermal annealing). This method is called a solid-phase growth method because conversion from an amorphous state to a crystal state is effected while the solid phase is maintained.
However, the silicon solid-phase growth has problems that the heating temperature and time need to be set at more than 600° C. and more than 10 hours, respectively, and that it is difficult to use an inexpensive glass substrate. For example, the Corning 7059 glass, which is commonly used in the active matrix liquid crystal display device, has a glass strain point of 593° C. Therefore, in view of increasing the substrate area, it is problematic to subject this glass to thermal annealing of more than 600° C. for a long time.
In connection with the above problems, studies of the present inventors have revealed that crystallization can be completed by performing heating at 550° C. for about 4 hours by depositing, by a very small amount, a certain kind of metal element such as nickel or palladium on the surface of an amorphous silicon film and then heating it. Naturally a silicon film even higher in crystallinity can be obtained by annealing of 600° C. and 4 hours.(refer to Japanese Unexamined Patent Publication No. Hei. 6-244103 (JP-A-6-244103).
To introduce a very small amount of metal element (for accelerating crystallization), various methods are available such as depositing a coating of a metal element or its compound by sputtering (refer to JP-A-6-244104), forming a coating of a metal element or its compound by such a means as spin coating (JP-A-7-130652), and forming a coating by decomposing a gas containing a metal element by thermal decomposition, plasma decomposition (JP-A-7-335548), or the like. Selection may be made properly among those methods in accordance with their features.
It is also possible to introduce a metal element selectively, i.e., into a particular portion, and then cause crystal growth to proceed from the portion where the metal element is introduced to the periphery by heating (called “lateral growth method”). Having directivity in crystallization, a crystalline silicon film produced by this method exhibits much superior characteristics when used properly in connection with the directivity.
It is also effective to further improve the crystallinity by performing illumination with strong light such as laser light after the crystallization step using a metal element (JP-A-7-307286). It is also effective to perform thermal oxidation after execution of the above-mentioned lateral growth method (JP-A-7-66425).
By performing crystallization by using a metal element in the above-described manner, a crystalline silicon film having better quality can be obtained at a lower temperature in a shorter time. The temperature of the heat treatment, which strongly depends on the kind of amorphous silicon film, is preferably set at 450°-650° C., even preferably at 550°-600° C.
However, the most serious problem of the above method is necessity of removing the introduced metal element. Adverse effects on the electrical characteristics and reliability of a metal element that is introduced in a silicon film are not negligible. In particular, because of the mechanism of the crystallization using a metal element, the metal element remains in the coating mainly as a conductive silicide and hence becomes a major cause of defects.
It is known that in general metal elements (particularly nickel, palladium, platinum, copper, silver, and gold) can be captured by crystal defects, phosphorus, etc. For example, JP-A-8-330602 discloses a technique for reducing the concentration of metal elements in a channel forming region by implanting phosphorus ions into a silicon film with a gate electrode as a mask and then performing thermal annealing (furnace annealing) or optical annealing (laser annealing or the like), thereby allowing metal elements that are contained in the silicon film to move to source and drain regions and fixing (gettering) the metal elements there.
In the technique of JP-A-8-330602, when phosphorus is implanted into the source and the drain, the silicon film is rendered amorphous and the number of crystal defects increases, whereby metal elements are gettered by phosphorus and crystal defects. The region where to implant phosphorus is not limited to the source and the drain, and may be any region except at least a region where a channel forming region is to be formed. It is apparent to a person having ordinary skill in the art that metal elements can be removed though the degree of removal depends on the distance from the phosphorus-implanted region.
To enable the gettering, annealing needs to be performed for a sufficient time for metal elements to move to the phosphorus-implanted region. Therefore, thermal annealing is suitable for this purpose. However, in general the annealing temperature effective for the gettering is more than 600° C. (it depends on the kind of metal element). Executing a process of such a high temperature for a long time increases a possibility that the substrate is deformed and hence becomes a factor of causing mask misalignment in later photolithography steps.
On balance, optical annealing is preferable. JP-A-8-330602 does not discuss a light source of the optical annealing and the embodiments include a statement that an excimer laser is used. However, the pulse width of excimer lasers is shorter than 100 nsec and experiments have shown that light illumination in such a short time cannot provide a sufficient gettering effect.